Friction drag and pressure drag are persistent problems in aerodynamic and other types of fluid flow design. Friction drag results primarily from the force of friction between a surface such as a wing or a fuselage section and the air or other fluid found within the boundary layer adjacent to that surface. When the fluid flow past the surface is laminar, the effect of friction drag is relatively small. However, when a turbulent fluid flow passes over the surface, the frictional drag force is typically much larger than with the laminar flow. With respect to modern aircraft, the frictional drag component can account for 50% or more of the total drag force experienced by such aircraft. Similarly, other aerodynamic or hydrodynamic structures (e.g., cars, ships, rockets, etc.) may experience large frictional drag forces due to turbulent flow passing over their external surfaces.
A second type of drag occurs when a flow over an aerodynamic or other surface separates from that surface to create a low pressure pocket behind the surface. Such flow separation may be caused when the aerodynamic surface interacts with the flow at a high angle of incidence or "angle of attack." The resulting low pressure pocket creates a retarding force and is commonly referred to as pressure drag. An energized or turbulent flow is less likely to become separated from an aerodynamic surface than a non-energized or laminar flow. Thus, one method of reducing pressure drag is to artificially convert or "trip" the laminar fluid flow over the surface to a turbulent flow. The energy within the turbulent boundary layer helps to maintain the flow attached to the surface, thereby reducing or delaying flow separation until a higher angle of attack so that a reduction in the total amount of pressure drag is achieved. However, the tripped turbulent flow will, in turn, contribute to a higher degree of friction drag, as discussed above.
Many prior methods have been used to reduce both friction and pressure drag. With respect to pressure drag, some of these methods include adding structure to the leading edge of an aerodynamic surface. Such structures may include rough strips extending spanwise along the leading edge of the surface or a plurality of vortex generators spaced along the leading edge. These structures extend into the relatively thin laminar boundary layer to disrupt the laminar flow, thereby prematurely tripping the flow to a turbulent state and energizing the boundary layer so that the flow is less likely to separate from the surface. While these and other similar structures may successfully reduce the pressure drag associated with flow separation, they do not address the resultant increase in friction drag caused by the larger proportion of turbulent flow within the boundary layer.
With respect to friction drag, a turbulent boundary layer has a greater velocity gradient than a laminar boundary layer, and the greater velocity gradient, combined with the inherent instability within the turbulent boundary layer, tends to transfer a relatively high amount of momentum from the boundary layer to the aerodynamic surface. Prior means for reducing friction drag have included both passive and active techniques for reducing the instability or the momentum transfer within the turbulent boundary layer. Examples of the passive control means include riblets formed on the aerodynamic surface or large eddy breakup units (LEBUs). With respect to riblets formed in the streamwise direction on the aerodynamic surface, the streamwise grooves formed by the riblets attempt to redirect the streamwise fluid flow within the boundary layer away from the aerodynamic surface, thereby reducing the momentum transfer between the boundary layer and the surface. However, while such passive devices have demonstrated that they are capable of reducing friction drag, the net effects of such devices are lessened due to offsetting drag increases in other areas. For example, while riblets may decrease the effect of friction drag, they also increase the wetted surface area of the aerodynamic surface so that the total amount of friction drag is not dramatically decreased. Additionally, the parameters of the riblets are not easily changed once they are optimized for a particular flight condition. Similarly, LEBUs contribute extra form or device drag to the total drag of the aerodynamic surface. One example of an active form of friction drag control is a suction system in which a pattern of fine holes is formed in the aerodynamic surface. Suction is applied to the holes to create a pressure gradient that suppresses instability growth within the turbulent boundary layer. However, the obvious drawbacks of such a system include its cost, ongoing maintenance and its susceptibility to adverse weather conditions.
Additionally, in a 1992 article entitled "Suppression of Turbulence in Wall-Bounded Flows by High-Frequency Spanwise Oscillations," Jung et al. utilized computational fluid dynamics simulations to determine whether a reduction in turbulence-induced drag could be realized in a simulated bounded channel flow by rapidly oscillating one of the channel walls in a spanwise direction (orthogonal to the direction of the simulated free stream channel flow). The article notes that the turbulent bursting process was suppressed and significant reductions in the calculated turbulent drag force were realized. However, Jung et al. offered no explanation or suggestion of how the spanwise oscillations could be achieved outside the purely computational realm.
A practical technique is needed for reducing the friction effects of turbulent boundary layers while not simultaneously contributing to other types of drag. Additionally, the technique for reducing friction drag would provide further value if it could reduce pressure drag by energizing laminar boundary layers while simultaneously working to reduce momentum transfer within the turbulent boundary layer.
It is with respect to these and other background considerations, limitations and problems, that the technique of the present invention has evolved.